you can use sulfuric acid to ph down. but keep in mind 1. highly reactiv at 98%!!! 2. it will add alot of sulfur to your nutrient profil. normaly you would call sulfur an element of freedom because our favorite plant can take sulfur quite good( that may depends on strain).
3. get a ph meter 
or rent one . calculating ph is working if you are a chemist in some way or a math man.
i would recommend to use phosporic acid 59% and/or nitric acid 3% ( this will also add N and P to the nutrient profile ) instead of sulfuric acid,
hope that helps and itΕ not confusing too much. if you have further questions drop me a pm i`ll try to help 